A device for direct solid-state conversion of nuclear energy to electrical energy, and, more particularly, a beta-cell that uses icosahedral boride compounds for the direct solid-state conversion of beta-particle energy to electrical energy.
Nuclear energy of some radioisotopes is primarily released in the form of beta particles. Beta particles are very energetic electrons that are emitted from a nucleus as a product of its decay. Prominent beta-emitting radioisotopes include 90Sr, 147Pm, and 170Tm. The beta particles emitted by these isotopes have maximum energies between 0.2 and 2.3 MeV. Beta cells utilize beta, particles from radioisotope decays with a material, such as a semiconductor material, to produce electrical power.
Upon passing through a semiconductor, a beta particle excites electrons thereby creating many electron-hole pairs. Each beta particle generates approximately 103 to 105 electron-hole pairs. The local electric field of a semiconductor junction tends to separate the paired electrons and holes thereby creating a current. This charge separation is especially efficient (near 100%) in materials in which the created electrons and holes each move with a high mobility (for example, greater than 10 cm2/V-sec at 300 K). Incident beta particles can thereby generate currents in a semiconductor junction. This current generation scheme is analogous to that by which incident photons create a current in a solar cell.
Devices that convert beta radiation directly to electricity are termed beta cells. Unlike solar cells, beta cell energy sources are self-contained and reliable. Unlike chemical power sources such as batteries, beta cells are not rapidly exhausted. Indeed, beta-cell development is motivated by the huge energy capacities of prominent beta sources (107 to 108 W-hr/kg of fuel) compared with those of even excellent chemical sources (e.g., 104 W-hr/kg of gasoline).
Beta cells can be designed to produce high power, to have a long half-life, or to require little shielding by choosing among different radioisotope energy sources. The size and mass of a beta cell will be determined primarily by the thickness of any shielding required to attenuate the Brehmsstrahlung and any gamma rays that accompany beta emission to desired levels. For example, the continuous power of emitted beta particles from 0.8 kg of the isotope 170Tm is about 10 kW. The half-life of 170Tm is about four months. Assuming 10% conversion efficiency, a beta cell fueled by this 170Tm source would deliver about 1 kW of electrical power for about four months. Shielding the Brehmsstrahlung and gamma rays from 0.8 kg of 170Tm to safe levels would require about 9 cm of lead. As a second example, the continuous power of emitted beta particles from 0.01 gm of 90Sr is about 10 mW over a half-life of 28 years. This small amount of 90Sr would require no shielding. Assuming 10% conversion efficiency, a beta cell fueled by this 90Sr source would continuously deliver about 1 mW of electrical power for tens of years.
Beta cells can find many applications wherever high-energy-capacity, reliable power sources are needed. For example, low-power beta cells could power remote sensors, microsystems, and small electronic appliances such as laptop computers and pacemakers. High-power beta cells could provide power for remote installations, spacecraft, and military units, among others.
Although beta cells have many potential uses, beta cells constructed with conventional semiconductors such as Si, Ge, GaAs, or CdTe have very limited utility because they suffer collateral radiation damage. In particular, incident high-energy beta particles create defects within the semiconductor that scatter and trap the generated charge carriers. This radiation damage accumulates, thereby degrading the performance of the semiconductor as an energy-conversion device. For example, silicon beta cells fueled by 90Sr were studied in the early 1950""s (see e.g., P. Rappaport, J. Loferski, and E. Linder, RCA Reviews, 1956, 17, 100-128). The electrical output of these cells degraded rapidly, over a few days, as a result of accumulating damage.
The output of conventional solar cells is degraded even by exposure to the very low flux of high-energy electrons encountered by orbiting satellites in space environments. The degree of degradation has been found to depend on the conventional semiconductor used in the solar cell (see e.g. Yamaguchi et al., U.S. Pat. No. 4,591,654, issued on May 27, 1986). The fluxes of energetic beta particles emitted by useful radioisotopes exceed the flux of energetic electrons in space by many orders of magnitude.
Thus, beta cells made of standard semiconductors such as Si can be used only for very short times or with very weak beta sources, such as 3H or 147Pm. Beta cells fueled by these very weak sources have been studied intermittently since the 1970""s (see e.g., T. Kosteski, N. Kherani, F. Gaspari, S. Zukotynski and W. Shmayda, J. of Vacuum Sci. and Tech. Part A, 1998, 16, 893-896; and L. Olsen, Proc. Of the 9th Intersociety Energy Conv. Eng. Conf., Amer. Nuclear Soc., 1974, 754-762).
Thermally insulated beta cells that would utilize heat from radioisotope decays to operate at high temperatures have been described (see Little et al., U.S. Pat. No. 5,260,621, issued on Nov. 9, 1993). It was proposed that high-temperature annealing of defects would limit the radiation-induced degradation of beta-cell performance.
Needed is a beta cell that can efficiently produce electricity at normal operating temperatures that have little or no radiation-induced degradation in performance.